A primary objective of a controller of an elevator bank is to optimize various performance measures according to constraints, such as minimize waiting time and travel time and maximize smoothness of travel, while at the same time balancing various cost measures related to operational expenses, such as minimizing total energy consumption, reducing equipment wear, and the like.
Most traction sheave elevators have counterweights. The counterweights ride on rails within the elevator shaft. The counterweights reduce significantly the peak power consumed. For an ideally balanced counterweight, the weight is equal to that of the car and the average number of passengers in the car, e.g., 40% capacity. Electric power is used only to accelerate and decelerate the car and to counteract friction.
Without a counterweight, the power consumed to move the car and the passengers increases significantly. However, when moving downwards, the weight of the car and passengers can be used to produce energy by means of regenerative braking. Therefore, the total energy consumption for elevators with and without counterweights is not necessarily very different. It is only the peak power consumption that differs significantly in the two cases.
Therefore, minimizing peak power consumption is of particular significance for traction sheave elevators without counterweights. Because energy and power are often confused, herein energy and power are distinguished and defined as follows: peak power is measured in watts (W); and energy consumption is measured in joules (J). That is, power is measured instantaneously, while energy is equal to power integrated over time.
U.S. Pat. No. 7,032,715, “Methods and apparatus for assigning elevator hall calls to minimize energy use,” issued to Smith et al. on Apr. 25, 2006 describes a method for minimizing total energy. This has no relationship to minimizing peak power at any instant in time. The invention solves this problem.
Higher peak power consumption usually results in larger and more expensive electrical equipment, such as thicker cables and larger transformers. On the other hand, eliminating the counterweights frees up useful space on every floor of the building, resulting in major savings for the owners of that building. Consequently, it is desirable to find a way to control the operation of a bank of elevators without counterweights so that the peak power consumption remains below a predetermined threshold value at all times.
Most elevator controllers operate by considering a large number of candidate schedules, i.e. assignments of hall calls to elevator cars. The controller selects the schedule that is optimal with respect to a set of constraints. Peak power control depends critically on the ability of the controller to determine quickly the peak power that would be consumed while operating according to a particular schedule. The schedule for an individual car includes a list of passengers assigned to this car, and the overall schedule for the bank of elevators includes all individual car schedules.
Because energy consumption is additive, it is sufficient to determine the energy consumption for each car as a function of time, and sum up the values for each time instant for all cars. There are several possible ways to compute the individual energy consumption of each car over time for a specific schedule. The simplest way assumes that the energy consumption is a constant that depends only on the direction of motion of the car.
Clearly, the value of this constant is much higher for upward motion than for downward motion. When regenerative braking is used to produce energy, in fact, the constant for downward motion can even be negative. In such case, determining the iota energy consumption is very simple, and reduces to counting of how many cars move in the upward direction and how many cars are moving in the downward direction. If too many cars are moving in the upward direction, in comparison to a suitably selected threshold, then the schedule can be rejected and excluded from consideration. That method is described in International Patent Publication WO 2006/095048 applied for by Kone Corporation, and U.S. Patent Application 20060243536, “Method and device for controlling an elevator group, published Nov. 2, 2006.
However, in practical applications, the energy consumption of an elevator car is not constant over time, because its speed and acceleration vary during interfloor flight according to optimal control constraints, so that the flight is completed in minimal time while providing comfortable travel for the passengers in the car.
Another Kone method described in the same application WO 2006/095048 estimates the energy consumption based on a simulation of the car flight to its destination floor. That method is feasible only if the path taken by the car during the execution of its schedule is completely known. This is possible if, for example, the passengers can indicate their destination when calling a car, instead after entering the car as is done normally. In such a case, there would be no uncertainty, the car path is completely known, and can be simulated by a simulator in real time.
However, most hall calls are indicated by the usual pair of up/down buttons. Thus, the exact destination floor is usually not known. The exception being hall calls generated at the penultimate floor in either direction. Otherwise, the destination can be any floor in the requested direction of service. Depending on the exact destination of each passenger, the car can take a large number of possible paths, and simulating all paths is not feasible. More specifically, the number of such paths is exponential in the size of the building and the number of waiting passengers. If a building has N floors and M calls are assigned to the car, then O(NM) possible paths need to be considered to compute the peak power consumption for all cars for any schedule.
This difficult problem is solved by the invention.